Internal combustion engines may operate in a variety of combustion modes. One example mode is homogeneous charge compression ignition (HCCI), wherein a homogeneous or near homogeneous air and fuel mixture achieves a temperature and/or pressure where combustion occurs by auto-ignition without requiring a spark being performed by a sparking device. Auto-ignition timing may be varied by adjusting one or more of the initial charge temperature, amount of residual or recirculated exhaust gas, actual and/or effective compression ratio, etc. While HCCI may have greater fuel efficiency and reduced NOx production compared to other combustion modes under selected conditions, it may also be limited since a substantial amount of heat and pressure may be needed to produce combustion. Further, transitions into and out of the HCCI mode may present various control issues. In general, HCCI combustion may be enhanced over a wider operating range by increasing the compression ratio of the cylinders. However, HCCI operation and similar modes of operation, such as PCCI (Premixed Charge compression ignition, where there is stratification of the air/fuel mixture) may be limited to low loads (torque).
Another example mode that may be used is spark ignition (SI) combustion. SI combustion may be used during high load and/or transient operating conditions to increase torque output and maintain sufficient control of combustion timing. Further, adjustments in spark ignition timing may be used to mitigate undesirable engine operating conditions, such as sustained engine knock. However, while spark retard may mitigate engine knock, this can reduce engine fuel efficiency. As such, engines may need to operate at lower actual or effective compression ratios during SI combustion to avoid knock.
Thus, a general incompatibility may exist between the higher compression ratio preferred for HCCI operation and the lower compression ratio for SI operation, limiting the benefits that can be achieved. While some adjustment to compression ratio may be available via variable valve timing, variable valve lift, variable compression ratio, etc., the degree of adjustment necessary may require significant amounts of additional engine hardware and increased cost. Further, even if such hardware is present, the ability to rapidly and effectively control both knock and compression ignition may be limited by the time characteristics of the mechanical device transitions, etc.
As such, the inventors herein have recognized that the above issues may be addressed in one example by a method of operating an engine, comprising performing homogeneous charge compression ignition combustion during a first operating condition; and performing spark ignition combustion during a second operating condition, where an amount of alcohol in at least one of said homogeneous charge compression ignition combustion and said spark ignition combustion is varied in response to at least an operating parameter of the engine. Directly injected alcohol may be a particularly advantageous way to control operation because of the large amount of knock suppression that results from evaporative cooling of the fuel/air charge. This can be used to enable operation at substantially higher compression ratio in spark ignition engines.
In this way, for example, it may be possible to utilize higher compression ratio engine operation without knock limitations when the engine operates under SI combustion mode, thereby enabling an expanded HCCI design range (i.e., increased compression ratio) and improved spark ignition fuel economy. Likewise, the inventors herein have also recognized that it is also possible to utilize adjustments in a relative amount of alcohol or a variable timing of alcohol delivery to control the timing of HCCI combustion, thereby further enabling improved operation and transient performance.
Note that there are various adjustments to an amount of alcohol that may be used, such as adjusting a ratio of an amount of alcohol to an amount of another fuel (e.g., gasoline), adjusting an absolute amount of alcohol, adjusting an alcohol concentration, and/or combinations thereof.